This invention relates to detachable handles and in particular to detachable handles used on blackboard chalk erasers.
Blackboard erasers are well known in the art. The typical eraser consists of rectangular strips of felt material having their edges bonded to a backing plate.
The backing plate is formed of thin, rigid material of a rectangular shape and the unbonded edges of the adjacent felt strips extend perpendicular from the backing plate. The user holds the backing plate and wipes the unbound edges of the felt strips across the blackboard to erase previous writing. The strips retain the chalk dust until subsequently cleaned by tapping or pounding the eraser against a firm surface. Many improvements have been made to the blackboard eraser with the object of retaining more chalk dust and thereby increasing the time between cleanings.
U.S. Pat. No. 4,007,509 to Odhner utilized rows of fiberglass filaments as a substitute for the felt strips. The filament rows were attached to a backing plate in a manner leaving gaps between the filament rows. These gaps or channels retained the chalk dust until cleaning.
U.S. Pat. No. 3,864,778 to Vopat, et al. comprised a unitary backing plate and erasing surface. This unitary eraser was formed by cutting a thermo plastic foam with a hot wire which sealed the surface pores of the foam. Subsequent abrading of the erasing surface opened chalk dust retaining pores.
U.S. Pat. No. 3,748,684 to Fraser used a nonreticulating, flexible polyester urethane foam with specific pore size to increase eraser chalk dust retention characteristics. This material was glued to a flat, channel shaped backing plate.
Not withstanding these improvements, the felt type eraser is by far the most prevalent chalkboard eraser. However, a number of problems characterize the use of this type of eraser.
To retain the form of the eraser, the length which the felt strips extend from the backing plate must be limited. Consequently, the user's hand which is gripping the backing plate is in close proximity to the blackboard during erasing. Chalk dust which is not retained by the eraser is deposited on the user's hand and is soon transferred to the user's clothing.
The proximity of the user's hand to the blackboard has another major drawback. The user's fingernails may come in contact with the blackboard during eraser movement. This contact results in a familiar and extremely objectional piercing screech being emitted as the fingernails scrape along the blackboard surface. In addition, painful injury to the fingernails and hands may result.
Yet another objectional feature of the common felt type eraser is its tendency to trip during use. When considerable pressure is applied to the eraser while wiping the blackboard, a substantial friction is developed between the eraser and the blackboard. This friction occurs where the eraser surface contacts the blackboard and results in a force on the eraser resisting the wiping movement. Concurrently, a force in the direction of wiping is applied by the user at the backing plate which is a short distance from the blackboard. These forces combine to rotate the eraser around its leading edge causing it to flip out of the user's grasp with considerable force. This action spreads retained chalk dust throughout the vicinity and on the user's clothes. In addition, the sudden tripping action can cause injury to the user's hand should it strike the blackboard.